// 链表

function defauleEquals (a, b) {
    return a === b;
}

function Node(el) {
    this.element = el;
    this.next = undefined;
}

class LinkedList {
    constructor(equalsFn = defauleEquals) {
        this.count = 0;
        this.head = undefined;
        this.equalsFn = equalsFn;
    }

    push (element) {
        const node = new Node(element);
        let current;

        if (this.head === null) {
            this.head = node;
        } else {
            current = this.head;
            while (current.next !== null) {
                current = current.next;
            }
            current.next = node;
        }
        this.count++;
    }

    insert (element, index) {
        if (index > 0 && index <= this.count) {
            const node = new Node(element);
            if (index === 0 ) {
                const current = this.head;
                node.next = current;
                this.head = node;
            } else {
                const previous = this.getElementAt(index - 1);
                const current = previous.next;
                node.next = current;
                previous.next = node;
            }
            this.count ++;
            return true;
        }
        return false;
    }

    getElementAt(index) {
        if (index > 0 && index <= this.count) {
            let node = this.head;
            for (let i = 0; i < index && node != null; i++) {
                node = node.next;
            }
            return node;
        }
        return;
    }

    remove(element) {
        const index = this.indexOf(element);
        return this.removeAt(index);
    }

    indexOf(element) {
        let current = this.head;
        for (let i = 0; i < this.count && current !== null; i++) {
            if (this.equalsFn(element, current.element)) {
                return i
            }
            current = current.next;
        }
        return -1;
    }

    removeAt(index) {
        if (index > 0 && index < this.count) {
            let current = this.head;
            if (index === 0) {
                this.head = current.next;
            } else {
                // let previous;
                // for (let i = 0; i < index; i++) {
                //     previous = current;
                //     current = current.next;
                // }
                // previous.next = current.next;
                const previous = this.getElementAt(index - 1);
                current = previous.next;
                previous.next = current.next;
            }
            this.count --;
            return current.element;
        }
        return;
    }

    getHead() {
        return this.head;
    }

    isEmpty() {
        return this.size() === 0;
    }

    size() {
        return this.count;
    }

    toString() {
        if (this.head === null) {
            return '';
        }
        let objString = `${this.head.element}`;
        let current = this.head.next;
        for (let i = 1; i < this.size() && current !== null; i++) {
            objString = `${objString}, ${current.element}`
        }
        return objString
    }
}